This invention generally relates to stayrods, and more particularly relates to a stayrod arrangement for enhancing removal of sludge from nuclear heat exchangers.
Although stayrods for use in nuclear heat exchangers are known in the prior art, these stayrods have a number of problems associated with them that interfere with sludge removal operations. However, before these problems can be appreciated, some background is desirable as to the structure and operation of a typical nuclear heat exchanger and its associated stayrods.
In this regard, a typical nuclear heat exchanger or steam generator generates steam when heat is transferred from a heated and radioactive primary fluid to a non-radioactive secondary fluid of lower temperature. The primary fluid flows through a plurality of U-shaped tubes, which are received through holes in a plurality of spaced-apart support plates and also through holes in a tubesheet disposed in the heat exchanger. The secondary fluid is caused to surround the exterior surfaces of the tubes as the primary fluid flows through the tubes. The walls of the tubes function as heat conductors for transferring heat from the heated primary fluid to the secondary fluid. Thus, as the primary fluid flows through the tubes, it gives up its heat to the secondary fluid surrounding the exterior surfaces of the tubes to produce steam that is used to generate electricity in a manner well known in the art.
Because the primary fluid is radioactive, the nuclear heat exchanger is designed such that the radioactive primary fluid flowing through the tubes does not commingle with and radioactively contaminate the nonradioactive secondary fluid surrounding the exterior surfaces of the tubes. Radioactive contamination of the secondary fluid is undesirable for safety reasons. Therefore, the tubes are designed to be leak-tight so that the radioactive primary fluid remains separated from the nonradioactive secondary fluid to avoid commingling the primary fluid with the secondary fluid. However, the previously mentioned support plates are subject to deflection or bowing during postulated accident scenarios (e.g., main streamline break). Although highly unlikely, such deflection or bowing conceivably may cause the support plates to wear against and conceivably breach the walls of the tubes extending through the holes of support plates, which breach in turn may cause the radioactive primary fluid to commingle with the nonradioactive secondary fluid. Therefore, a plurality of elongate stayrods interconnect the support plates with the tubesheet to prevent deflection or bowing of the support plates. In addition, each stayrod is surrounded by a tubular spacer interposed between adjacent support plates to assist in maintaining the support plates in their nondeflected spaced-apart relationship.
Moreover, the secondary fluid entering the heat exchanger may contain suspended particles that precipitate-out of the secondary fluid and accumulate on the surface of the tubesheet as sludge. This accumulation of sludge is undesirable because it can lead to thinning of the walls of the tubes, which thinning conceivably may ultimately cause a breach of the tube walls allowing the primary fluid to commingle with the secondary fluid, a highly undesirable result. Such thinning may preferentially occur near the elevation of the tubesheet at a height corresponding to the height of sludge that has accumulated on the tubesheet. The sludge itself is primarily a combination of iron oxides and copper compounds together with trace amounts of other metals that have settled-out of the secondary fluid and onto the tubesheet. It is believed by applicant that such sludge deposits on the tubesheet may provide sites for concentration of phosphates or other corrosive agents at the tube wall in amounts sufficient to cause the previously mentioned tube wall thinning.
Therefore, such nuclear heat exchangers are typically serviced during shutdown periods to remove the sludge deposits on the tubesheet. Servicing of the heat exchanger involves opening inspection port covers located near the elevation of the tubesheet and inserting a fluid lancing tool therethrough, which fluid lancing tool emits at least one jet of high pressure fluid into the respective lanes defined between the tubes for dislodging the sludge from the tubesheet surface. In this regard, the fluid lancing tool is moved along the tubesheet while the jet of high pressure fluid is directed perpendicularly with respect to the movement of the fluid lancing tool. The high pressure fluid entrains the sludge in a flow of fluid which is then suctioned from the tubesheet. Such a sludge removal system is more fully described in U.S. Pat. No. 4,079,701 titled "Steam Generator Sludge Removal System" issued Mar. 21, 1978 in the name of Robert A. Hickman, et al. and assigned to the assignee of the present invention.
However, the previously mentioned prior art stayrods with their surrounding spacers may impede lancing some areas of the tubesheet because the spacers have a diameter larger than the tubes and therefore effectively block some of the fluid lancing lanes. Blocking of the fluid lancing lanes interferes with removing a portion of sludge from the heat exchanger. This is undesirable because leaving sludge residue in the heat exchanger after servicing may ultimately lead to the previously mentioned tube wall thinning, which thinning in turn may ultimately lead to the radioactive primary fluid commingling with the nonradioactive secondary fluid. Consequently, a problem in the art is to remove as much sludge as possible from the tubesheet even in the presence of stayrods and spacers.
Stayrods facilitating sludge removal are known. Such a stayrod is disclosed in U.S. Pat. No. 4,777,911 titled "Stayrod Configuration For Facilitating Steam Generator Sludge Lancing" issued Oct. 18, 1988 in the name of Robert M. Wepfer and assigned to the assignee of the present invention. This patent discloses a stayrod configuration which facilitates fluid lancing of the tubesheet, the stayrod configuration including a plurality of stayrods, each having the same diameter as the tubing and being threaded into the tubesheet at spaced positions which match the tube pattern. Although the stayrod configuration disclosed in this patent allows the removal of a relatively large amount of sludge from the lanes defined by the tubes, its use is nonetheless less than completely satisfactory. That is, although the plurality of stayrods disclosed in this patent each has the same diameter as the tubes, each group of stayrods appear to present a relatively large cumulative transverse profile to the fluid emitted from the lance, which profile may undesirably interfere with the ability of the fluid lance to completely remove the sludge from the lanes.
Hence, although stayrods for facilitating sludge removal are known in the art, the prior art does not appear to disclose a stayrod arrangement for suitably enhancing removal of sludge from nuclear heat exchangers.
Therefore, what is needed is a stayrod arrangement for enhancing removal of sludge from nuclear heat exchangers.